The present invention concerns the field of multi-layered cells, i.e. cells comprising a plurality of superposed substrates joined in pairs by sealing frames delimiting as many sealed volumes for containing an optically active medium, or whose electric properties can change under the effect of an incident light.
In the simplest case, electro-optical cells such as liquid crystal cells, or electrochemical photovoltaic cells include only two substrates, namely a transparent front substrate and a back substrate that may or may not be transparent. A network of conductive elements is formed on these two substrates forming electrodes and conductive paths connecting the electrodes to connection pads. The two substrates are joined to each other by means of a sealing frame defining a sealed volume in which the active medium is trapped.
Electro-optical cells are display cells wherein the optical features of the liquid crystals trapped between the two substrates of such a cell can be modified by control voltages applied to the electrodes. Electrochemical photovoltaic cells are capable of converting visible light into electricity by exploiting the photoelectric effect, which appears in a semiconductor substrate sensitised by a dyeing agent.
In the simplest case where the cells include only two substrates and one layer of active agent trapped between the two, filling the cells does not raise any particular problems. A filling aperture is simply arranged in the sealing frames through which the active agent can penetrate the sealed volume delimited by said sealing frame. Filling is usually carried out in the following manner: after arranging the cell and the liquid crystal in a vacuum chamber, air is extracted from the cell containment volume, then the side of the cell where the filling aperture is located is plunged into a receptacle containing the active agent. The active agent penetrates the cell via capillary action, through the filling hole, helped by an increase in pressure in the chamber. When the cell has been filled, the filling aperture need only be hermetically sealed.
Another known solution consists in piercing a filling hole in one of the cell substrates. After having extracted the air, the active agent is injected through the hole into the cell and, after the cell has been filled, said hole is sealed. This second solution proves particularly advantageous insofar as it allows the cells to be filled from above, which means that one can work with sets of cells still in batches.
Whichever solution is used in order to fill the cells, it is clear that this step of manufacturing said cells does not raise any major problem, in particular because of the fact that only a single type of active agent is used. Consequently, even if the active agent, for example a liquid crystal, flows out of the filling hole and wets, particularly by capillary action, the periphery of the cell or the substrates, it is not liable to be polluted by another liquid crystal or damaging a structure of the cell that has already been deposited.
The same is not true however with cells having several levels which contain different active agents. In this case, the aforementioned problem becomes important quickly and the risk of polluting one of the active agents such as a liquid crystal by another liquid crystal is quite high.
Let us imagine the case, for example, of a liquid crystal display cell with two levels including two layers of different liquid crystals. Such a cell can be formed of an intermediate substrate that carries the electrodes on both of its faces and of two upper and lower end substrates. Each cell is thus formed by joining the upper substrate, respectively the lower substrate, to the intermediate substrate by means of a sealing frame that delimits a sealed cavity to contain the liquid crystals. These sealing frames each include a filling hole, the filling holes being preferably arranged on the same side of the cell. In order to fill the cell, two foam pads or stoppers saturated with the desired liquid crystal are then applied against the filling apertures, and then the different liquid crystals penetrate, via capillary action, their respective cavities. After removing the stoppers, the filling apertures need only be hermetically sealed.
The method described hereinbefore advantageously allows the different levels of a multi-layered cell to be simultaneously filled with liquid crystals, which can vary from one layer to the next. However, the limits of this method are quickly discovered. Indeed, in order to implement this method, it is still necessary to be able to have at least one side of the cell available, free of any connection pad insofar as these connection pads, which are used to connect the cell electrodes to an external control circuit, are usually arranged along one edge of said cell where the lower substrate projects with respect to the upper substrate. Thus, it would be difficult to access a filling hole arranged in the sealing frame at a location where the connection pads appear. The more levels a cell has, the more the number of its electrodes and thus its connection pads increases. One could thus have a situation in which the connection pads open out along every sides of a cell, such that the filling technique described hereinbefore can no longer be used.